The terahertz spectral range (lambda = 30-300 mu m) has long been devoid of compact, electrically pumped, room-temperature semiconductor sources(1-4). Despite recent progress with terahertz quantum cascade lasers(2-4), existing devices still require cryogenic cooling. An alternative way to produce terahertz radiation is frequency down-conversion in a nonlinear optical crystal using infrared or visible pump lasers(5-7). This approach offers broad spectral tunability and does work at room temperature; however, it requires powerful laser pumps and a more complicated optical set-up, resulting in bulky and unwieldy sources. Here we demonstrate a monolithically integrated device designed to combine the advantages of electrically pumped semiconductor lasers and nonlinear optical sources. Our device is a dual-wavelength quantum cascade laser(8) with the active region engineered to possess giant second-order nonlinear susceptibility associated with intersubband transitions in coupled quantum wells. The laser operates at lambda(1) = 7.6 mu m and lambda(2) = 8.7 mu m, and produces terahertz output at lambda = 60 mu m through intracavity difference-frequency generation.
A time-resolved mid-infrared upconversion technique based on sum-frequency generation was applied to measure pulse propagation in lambda similar to 5.0 mu m quantum cascade lasers operated in continuous wave at 30 K. The wavelength-dependent propagation delay of femtosecond mid-infrared pulses was measured to determine the total group-velocity dispersion. The material and waveguide dispersion were calculated and their contributions to the total group-velocity dispersion were found to be relatively small and constant. The small-signal gain dispersion was estimated from a measurement of the electroluminescence spectrum without a laser cavity, and was found to be the largest component of the total GVD. A negative group-velocity dispersion of beta(2) ( = d(2)beta/d omega(2)) approximately -4.6 x 10(-6) ps(2)/mu m was observed at the peak emission wavelength, and good agreement was found for the measured and calculated pulse-broadening. (c) 2007 Optical Society of America.
We present a systematic study of the current - voltage characteristics and electroluminescence of gallium nitride ( GaN) nanowire on silicon ( Si) substrate heterostructures where both semiconductors are n- type. A novel feature of this device is that by reversing the polarity of the applied voltage the luminescence can be selectively obtained from either the nanowire or the substrate. For one polarity of the applied voltage, ultraviolet ( and visible) light is generated in the GaN nanowire, while for the opposite polarity infrared light is emitted from the Si substrate. We propose a model, which explains the key features of the data, based on electron tunnelling from the valence band of one semiconductor into the conduction band of the other semiconductor. For example, for one polarity of the applied voltage, given a sufficient potential energy difference between the two semiconductors, electrons can tunnel from the valence band of GaN into the Si conduction band. This process results in the creation of holes in GaN, which can recombine with conduction band electrons generating GaN band-to-band luminescence. A similar process applies under the opposite polarity for Si light emission. This device structure affords an additional experimental handle to the study of electroluminescence in single nanowires and, furthermore, could be used as a novel approach to two- colour light-emitting devices.
We demonstrate a compact, single-mode quantum cascade laser source continuously tunable between 8.7 and 9.4 mu m. The source consists of an array of single-mode distributed feedback quantum cascade lasers with closely spaced emission wavelengths fabricated monolithically on a single chip and driven by a microelectronic controller. Our source is suitable for a variety of chemical sensing applications. Here, we use it to perform absorption spectroscopy of fluids.
We present a novel technique for reliable electrical injection into single semiconductor nanowires for light-emitting diodes and lasers. The method makes use of a high-resolution negative electron-beam resist and direct electron-beam patterning for the precise fabrication of a metallic top contact along the length of the nanowire, while a planar substrate is used as a bottom contact. It can be applied to any nanowire structure with an arbitrary cross section. We demonstrate this technique by constructing the first zinc oxide single-nanowire light-emitting diode. The device exhibits broad sub-bandgap emission at room temperature.
We theoretically and experimentally illustrate a new apertured near-field scanning optical microscopy (NSOM) technique, termed differential NSOM (DNSOM). It involves scanning a relatively large (e.g., 0.3-2 mu m wide) rectangular aperture (or a detector) in the near-field of an object and recording detected power as a function of the scanning position. The image reconstruction is achieved by taking a two-dimensional derivative of the recorded power map. Unlike conventional apertured NSOM, the size of the rectangular aperture/detector does not determine the resolution in DNSOM; instead, the resolution is practically determined by the sharpness of the corners of the rectangular aperture/detector. Principles of DNSOM can also be extended to other aperture/detector geometries such as triangles and parallelograms.
In this paper we investigate the performance of quantum cascade (QC) lasers for high frequency modulation spectroscopy, particularly using frequency modulation (FM) and two-tone (2T) techniques. The coupling of the rf signal to the QC laser through the cryostat is studied in detail as well as the noise contributions of both the detector and the laser source to the final spectra. The experimental traces are obtained by spectroscopy on low-pressure N2O and CH4 gases at 8.0 mu m and 7.3 mu m wavelength, respectively, and reproduce the line profiles predicted by theory. As a preliminary result, an enhancement of a factor six is measured with respect to direct absorption line recording.
High-power quantum cascade lasers (QCLs) working in continuous wave (cw) above 400 K are presented. The material was grown by low-pressure metal organic vapor-phase epitaxy and processed into narrow buried heterostructure lasers. A cw output power of 204 mW was obtained at 300 K with an 8.38 mu m wavelength, 3 mm long and 7.5 mu m wide coated laser. The device operates in cw mode above 400 K, which exceeds the previous maximum cw temperature operation of QCLs by approximately 60 K. Preliminary reliability data obtained by accelerated aging tests indicate a remarkable robustness of the lasers. (c) 2006 American Institute of Physics.
The authors report the fabrication of high-power strained quantum cascade lasers working in continuous mode above 370 K. The devices, processed in narrow buried heterostructures, were grown by low-pressure metal organic vapor-phase epitaxy. Continuous wave output power as high as 312 mW at 300 K was obtained at a wavelength of 5.29 mu m from a 3.25 mm long, 7.5 mu m wide laser with a high-reflectivity back facet coating. The slope efficiency was in excess of 1.5 W/A and the power conversion efficiency reached almost 5%.
We report a hybrid approach for photonic systems that combines chemically synthesized single nanowire emitters with lithographically defined photonic crystal and racetrack microresonator structures. Finite-difference time-domain calculations were used to design nanowire photonic crystal structures where the photonic band gap overlaps the electronic band gap of the nanowire. Photoluminescence (PL) images of cadmium sulfide (CdS) nanowire photonic crystal structures designed in this way demonstrate localized emission from engineered defects and light suppression in regions of the photonic crystal. PL spectroscopy studies of defect-free nanowire photonic crystal structures further demonstrate the photonic band gap or stop band that spans most of the CdS band edge emission spectrum. In addition, single CdS nanowire-racetrack microresonator structures were fabricated, and PL imaging and spectroscopy measurements show good coupling of the nanowire to the microcavity including efficient feedback and amplified spontaneous emission. These hybrid structures exploit unique strengths of bottom-up and top-down approaches and thereby open new opportunities in nanophotonics from efficient and localized light sources to integrated optical processing.
In this Letter, we report the tuning of the emission wavelength of a single mode distributed feedback quantum cascade laser by modifying the mode effective refractive index using fluids. A fabrication procedure to encapsulate the devices in polymers for microfluidic delivery is also presented. The integration of microfluidics with semiconductor laser ( optofluidics) is promising for new compact and portable lab-on-a-chip applications. (c) 2006 Optical Society of America.
The authors have demonstrated a surface plasmon device composed of a resonant optical antenna integrated on the facet of a commercial diode laser, termed a plasmonic laser antenna. This device generates enhanced and spatially confined optical near fields. Spot sizes of a few tens of nanometers have been measured at a wavelength similar to 0.8 mu m. This device can be implemented in a wide variety of semiconductor lasers emitting in spectral regions ranging from the visible to the far infrared, including quantum cascade lasers. It is potentially useful in many applications including near-field optical microscopes, optical data storage, and heat-assisted magnetic recording. (c) 2006 American Institute of Physics.
We present the pulsed operation at room temperature of different strained InGaAs/AlInAs quantum-cascade lasers grown by low-pressure metalorganic vapor-phase epitaxy. Devices based on a bound-to-continuum transition design have threshold current densities in pulsed mode as low as 1.84 kA/cm(2) at 300 K. Identical lasers grown at higher rate (0.5 nm/s) also have threshold current densities lower than 2 kA/cm(2) at 300 K. Buried heterostructure lasers based on a double phonon resonance design were operated in continuous mode up to 280 K. Overall, the performance obtained from strained quantum cascade lasers deposited by metalorganic vapor-phase epitaxy are comparable with that of similar structures grown by molecular beam epitaxy. (c) 2006 American Institute of Physics.
Quantum fluctuations of the electromagnetic field give rise to a zero-point energy that persists even in the absence of electromagnetic sources. One striking consequence of the zero-point energy is manifested in the Casimir force, which causes two electrically neutral metallic plates to attract in order to reduce the zero-point energy. A second, less well-known, effect is a torque that arises between two birefringent materials with in-plane optical anisotropy as a result of the zero-point energy. In this paper, we discuss the influence of Brownian motion on two birefringent plates undergoing quantum electrodynamical ( QED) rotation as a result of the system's zero-point energy. Direct calculations for the torque are presented, and preliminary experiments are discussed.
We consider a gedanken experiment with a beam of atoms in their ground state that are accelerated through a single-mode cavity. We show that taking into account of the ``counterrotating'' terms in the interaction Hamiltonian leads to the excitation of an atom with simultaneous emission of a photon into a field mode. In free space, when the atom-field interaction is turning on/off adiabatically, the only nonadiabatic effect that causes the excitation is the time-dependent Doppler shift. The resulting ratio of emission and absorption probabilities is exponentially small and is described by the Unruh factor. In the opposite case of rapid turn on of the interaction on the cavity boundaries the above ratio is much greater and radiation is produced with an intensity which can exceed the intensity of radiation in free space by many orders of magnitude. In both cases real photons are produced. The cavity field at steady state has a thermal density matrix. However, under some conditions laser gain is possible. We present a detailed discussion of how the acceleration of atoms affects the generated cavity field in different situations. We identify a common physical mechanism behind the Unruh effect and similar QED radiation processes.
We present our most recent results seeking to understand the dependence of quantum fluctuations of the electromagnetic field on the dielectric properties of two boundary surfaces. In the first section, we provide a detailed description of our measurement of the skin-depth effect of the Casimir-Lifshitz force. The second section is devoted to the torque induced by quantum fluctuations on two birefringent plates.
We demonstrate a quasiphase matching scheme for second-harmonic generation in quantum cascade lasers with integrated resonant nonlinearity. Modulation of the nonlinear susceptibility is achieved by the periodic modulation of the bias voltage along the ridge waveguide leading to a periodic shift of electronic resonances and a change in the electron population in different energy levels. An up to tenfold enhancement of the conversion efficiency is observed. This technique is applicable to any resonant nonlinear optical process in quantum wells. (c) 2006 American Institute of Physics.
We report the controlled synthesis of axial modulation-doped p-type/intrinsic/n-type (p-i-n) silicon nanowires with uniform diameters and single-crystal structures. The p-i-n nanowires were grown in three sequential steps: in the presence of diborane for the p-type region, in the absence of chemical dopant sources for the middle segment, and in the presence of phosphine for the n-type region. The p-i-n nanowires were structurally characterized by transmission electron microscopy, and the spatially resolved electrical properties of individual nanowires were determined by electrostatic force and scanning gate microscopies. Temperature-dependent current-voltage measurements recorded from individual p-i-n devices show an increase in the breakdown voltage with temperature, characteristic of band-to-band impact ionization, or avalanche breakdown. Spatially resolved photocurrent measurements show that the largest photocurrent is generated at the intrinsic region located between the electrode contacts, with multiplication factors in excess of ca. 30, and demonstrate that single p-i-n nanowires function as avalanche photodiodes. Electron- and hole-initiated avalanche gain measurements performed by localized photoexcitation of the p-type and n-type regions yield multiplication factors of ca. 100 and 20, respectively. These results demonstrate the significant potential of single p-i-n nanowires as nanoscale avalanche photodetectors and open possible opportunities for studying impact ionization of electrons and holes within quasi-one-dimensional semiconductor systems.
We investigate the implementation of surface emission via a second order grating in terahertz quantum cascade lasers with double-metal waveguides. Absorbing edge structures are designed to enforce antireflecting boundary conditions, which ensure distributed feedback in the cavity. The grating duty cycle is chosen in order to maximize slope efficiency. Fabricated devices demonstrate surface emission output powers that are comparable to those measured from edge-emitting double metal waveguide structures without gratings. The slope efficiency of surface emitting lasers is twice that of double-metal edge emitting structures. Surface emitting lasers show single mode behavior, with a beam divergence along the laser ridge of approximately six degrees. (c) 2006 Optical Society of America.